X-ray powder diffraction used to evaluate the kinetics of various processes related to ordering in Ni_1+dSn (d = 0.35, 0.50)

A. Leineweber, E. J. Mittemeijer

Max Planck Institute for Metals Research, Heisenbergstraße 3, 70569 Stuttgart, Germany.

The binary system Ni-Sn contains a series of non-stoichiometric phases Ni_1+dSn with Ni₂In/NiAs type structures: a NiAs type sub-structure is formed by Ni(1)Sn (Sn taking the place of As) with additional Ni(2) atoms occupying partially the centres of the trigonal-bipyramids formed by five Sn atoms. In the hexagonal high-temperature (HT) phase with ca. 0.27 £ d £ 0.65 these Ni(2) atoms show no long-range order. Occupational ordering of Ni(2) is exhibited, however, by the three different but structurally similar orthorhombic low-temperature phases (LT, LT', LT'' [1-5]). The order-disorder phase-transition temperature is strongly composition dependent, e.g. the disordered HT-Ni_1.50Sn transforms to the ordered LT phase below 782 K, whereas for Ni_1.35Sn the LT’ phase forms below 686 K. The equilibrium phase transitions occur discontinuously, i.e. by a first order transition [2]. States of (dis)order can be retained at ambient temperatures by quenching the alloys in water.

The present paper reports X-ray powder diffraction investigations on non-equilibrium states of Ni_1+dSn obtained by various heat treatment procedures, providing insight into the mechanism of the different observed processes. For that three alloys were equilibrated at different annealing temperatures T₁ and subsequently quenched:

HT-Ni_1.50Sn quenched from T₁ = 1023 K

HT-Ni_1.35Sn quenched from T₁ = 1023 K

LT’-Ni_1.35Sn quenched from T₁ = 683 K.

These samples were afterwards annealed at various temperatures T₂ < 600 K for certain periods of time, quenched and then characterised by X-ray powder diffraction. The gradual changes from the equilibrium state quenched from T₁ to the new equilibrium state (often finally reached only after long times) at T₂ can be traced by measuring lattice parameters, integral reflection intensities and diffraction-line broadening as a function of the annealing time.

Annealing of HT-Ni_1.50Sn and HT-Ni_1.35Sn at T₂ generates new long-range order leading finally to the LT/LT’ phases in equilibrium at T₂. This order formation occurs in a two-step fashion, which involves firstly formation of long-range order in small domains, and secondly coarsening of the ordered domains as exhibited by a narrowing of the superstructure reflections (Figure 1).

The LT’-Ni_1.35Sn sample equilibrated at and quenched from T₁ = 683 K shows long-range order with sharp superstructure reflections. However, the degree of long-range order as obtained from the intensities of the superstructure reflections is distinctly lower than for samples equilibrated at lower temperatures. In this case annealing at various temperatures T₂ now reveals simply a gradual increase of the already present long-range order (Figure 2), in contrast with the two-stage order-formation process exhibited by the samples equilibrated at T₁ = 1023 K.

The X-ray powder diffraction data were used to evaluate the kinetics of the observed processes. Particularly, the activation energies were determined. Methods for such analyses were developed. The contribution of the activation-energy values to the understanding of the transformation mechanisms is discussed.

Figure 1: Coarsening of small-domain LT-Ni1.50Sn produced by annealing HT-Ni1.50Sn as observed by an evaluation of Scherrer-type broadening of the superstructure reflections (stage 2 of order formation).	Figure 2 Evolution of the ‘orthorhombic distortion’ used as a measure for the degree of long-range order in LT’-Ni1.35Sn equilibrated at T1 = 683 K and annealed subsequently for various times at various temperatures T2.

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